The worldwide need for energy requires that large amounts of fuel be transported from the areas in which they are found to the consumer. One of the cleanest and most abundant energy forms for practical use today is natural gas. The major gas fields are usually situated far from the main consuming markets and there is thus the need for transporting the gas to the consumer from the fields. Pipe line transportation is one of the possibilities which may be considered, however piping is quite expensive and unfeasible for long range transportation. Ship transportation thus remains the practical solution for gas transportation applications, in particular transportation of liquefied gas.
The present invention relates generally to liquefied gas tanks such as LNG tanks or LPG tanks, although the term cryogenic tank is used generally in this description. LNG is usually kept in the tank at the boiling point of about −163° C. at atmospheric pressure, constantly boiling off methane. In order to reduce the boil-off, the thermal influx through the tank wall may be sought reduced by arranging an insulation layer about the tank walls. The tank walls must be structurally supported and stabilized, but all such structurally supporting parts may conduct heat into the tank and thus induce undesired boil-off. It is thus desirable to reduce the total cross-section of structurally supporting parts extending through the insulation layer in order to reduce the thermal influx. A general problem with LNG tanks and other cryogenic tanks is the thermal contraction that takes place during the initial cooling and filling of the tank, and possibly the thermal expansion of the tank if the LNG is removed from the tank due to evaporation or by being emptied.
LNG tanks are often retrofitted onto previously built ships or tankers, but may also be directly installed on installations such as floating production and storage units (FPSO) and floating storage and regasification units (FSRU). For these applications simplicity of installation is critical to reduce cost, as is availability of deck space. There are furthermore a number of cryogenic tank applications onshore in industrial use. The various applications present different issues to be resolved, some of the main problems being the temperature, volatility and toxicity of the gases. A number of tank designs have been proposed for these applications all having advantages and disadvantages.
When performing a cryogenic filling process the bottom plate structure and the tank wall structure of a tank will contract when the tank cools from ambient temperature. The bottom plate structure and the lower part of the circumference of the tank wall structure will contract first, then, through thermal conductivity and through direct liquid and evaporated gas contact with the wall, the tank wall will cool and contract while the tank is filled with the liquid natural gas. Particularly for LNG ship tanks but also for some land tanks it is required that the tank is prevented from moving laterally relative to the substrate during cooling. For ship tanks this lateral stabilization is important during sailing. Cryogenic inner tanks need to be designed to tolerate the thermal contraction of the tank with respect to the supports. This occurs due to the low temperature of the cryogenic fluid which naturally will lower the temperature of the tank itself and the supports to which it is fixed. In addition to the contraction of the tank which occurs while filling the tank with the cryogenic fluid, there will be a corresponding expansion of the tank upon emptying the tank.
The differential thermal contraction may induce strains in the tank wall liner plate, the tank wall girder structure, and in the supporting compartment structure. Strains in the LNG tank liner plate may incur cracks that may result in leaking of LNG which is critical due to the risk of fire and explosion, and due to the toxicity of the methane. Tank breakage and resulting leakage of cryogenic gas in a vessel may furthermore result in the catastrophic loss of a vessel as the structural steel of such vessels is not designed for being exposed to such low temperatures.
For ships and other vessels one has a major issue in connection with sloshing of the LNG due to the action of waves upon the vessel or due to the vessel movement itself. Sloshing may induce failures in the tank, and the tank should thus be designed to withstand sloshing effects. The present invention describes a practical solution to some of the above-mentioned challenges.